﻿<p>This entity defines an action which is distributed over a surface.
A surface action may be connected with a surface member or surface connection.</p>

<blockquote class="history">
HISTORY&nbsp; New entity in IFC4.
</blockquote>

<blockquote class="change-ifc2x4">
IFC4 CHANGE&nbsp; Former entity <em>IfcStructuralPlanarActionVarying</em> from IFC2x2 has been removed and are replaced by this entity.
</blockquote>

<p><b><u>Coordinate Systems</u></b>:</p>

<p>See definitions at <em>IfcStructuralActivity</em>.</p>

<p><b><u>Topology Use Definitions</u></b>:</p>

<p>Standard Case:<br>
If connected with a surface item and acting on its entirety, instances of
<em>IfcStructuralCurveAction</em> shall not have an <em>ObjectPlacement</em> nor a
<em>Representation</em>.  It is implied that the placement and representation of the
<em>IfcStructuralActivity</em> is the same as the ones of the member or connection.</p>

<p>Special Case 1:<br>
If connected with a surface item but acting only on a part of it, instances of
<em>IfcStructuralSurfaceAction</em> shall have an <em>ObjectPlacement</em> and
<em>Representation</em>, containing an <em>IfcFaceSurface</em> which topologically
defines the loaded part of the surface.  See <em>IfcStructuralActivity</em> for further
definitions.</p>

<p>Special Case 2:<br>
If not connected with a structural item (which may happen in an incomplete or conceptual model),
a surface action should have an <em>ObjectPlacement</em> and <em>Representation</em>, containing an
<em>IfcFaceSurface</em>.  See <em>IfcStructuralActivity</em> for further definitions.</p>

<p class="spec-head">Informal Propositions:</p>

<ol>
  <li>If the surface action is of the predefined type CONST,
      <em>SELF\IfcStructuralActivity.AppliedLoad</em> must not be of type
      <em>IfcStructuralLoadConfiguration</em>.</li>
  <li>If the surface action is of the predefined type BILINEAR,
      <em>SELF\IfcStructuralActivity.AppliedLoad</em> shall be of type
      <em>IfcStructuralLoadConfiguration</em> and shall contain three items
      with two-dimensional <em>IfcStructuralLoadConfiguration.Locations</em>,
      defining the location of the load samples in local coordinates of the
      surface action.</em>
  <li>If the surface action is of the predefined type DISCRETE,
      <em>SELF\IfcStructuralActivity.AppliedLoad</em> shall be of type
      <em>IfcStructuralLoadConfiguration</em> and shall contain two or more
      items with two-dimensional locations.</em>
  <li>Point loads must be of type DISCRETE, thus contain two or more load points.
      (Single point loads are modeled by <em>IfcStructuralPointLoad</em>.)</li>
  <li>All items in <em>SELF\IfcStructuralActivity.AppliedLoad\IfcStructuralLoadConfiguration.Values</em>
      shall be of the same entity type.</li>
</ol>